The use of a single resistance wire formed into a helical coil for use in electric resistance heating either for heating moving air, for radiant heating, or for convection heating is well known in the prior art. In one type of heater, the resistance coils are energized to heat air passing over the coils, the heated air then being directed in a particular manner for heating purposes. One application using such a heater is an electric clothes dryer.
Examples of open coil heaters are found in U.S. Pat. Nos. 5,329,098, 5,895,597, 5,925,273, 7,075,043, and 7,154,072, all owned by Tutco, Inc. of Cookeville, Tenn. Each of these patents is incorporated by reference in its entirety herein. One type of an open coil electric resistance heater is a two stage heater described in U.S. Pat. No. 7,075,043. A side view of this type of heater is shown in FIG. 1 and designated by the reference numeral 10. The heater 10 has two heater elements 10a and 10b, optimally for use in a clothes dryer. The elements 10a and 10b are supplied with electricity via terminals 12 extending from the terminal block 28. The heater elements 10a, 10b are supported by a support plate 14, which in turn supports a plurality of support insulators 16, typically made of ceramic material and which are well known in the art. The support insulators 16 support and isolate coiled portions of the elements, 10a and 10b, during operation of the heater.
The heater 10 includes opposing sidewalls (one shown as 6 in FIG. 1), wherein projections in the plate 14 extend through slots 20 in the sidewall 6 to allow the sidewalls to support the plate.
Each of the electric heater elements, 10a and 10b, is arranged in series of electrically continuous coils which are mounted on the plate 14 in a spaced-apart substantially parallel arrangement. Each heater assembly 10a and 10b is arranged substantially equally and oppositely on both sides of the plate. Crossover portions 22a and 22b of each heater element 10a and 10b are provided wherein each crossover links one coil of each of the elements mounted on one side of the plate 14 with another coil of the same element found on the other side of the plate.
Electricity is supplied to the heater assembly through the terminal block 28. The heater elements, 10a and 10b, are arranged so that the terminal connector portions or wire leads 32 and 34 which extend from an end 38 of each of the mounted coil sections to the terminal block are as short as possible. This aids in eliminating or reducing the need for supporting the connector portions. For the longer runs, the wire leads, 32 and 34, are partially enclosed with an insulating member 36. The insulating member 36 may be formed from any type of insulating material suitable for this purpose, e.g., a ceramic type. The insulating member is generally tubular in shape and rigid.
Another type of heater manufactured by Tutco, described in U.S. patent application Ser. No. 11/987,542 (herein incorporated by reference) is an improvement over the heater shown in FIG. 1, in that the heater coils are parallel to air flow to minimize noise, prevent coil “shadowing, and promote heat transfer from the heater coils to the air stream.
In the manufacture of appliances and equipment, especially clothes dryer manufacture, that require open coil electric heaters mounted in an air duct to heat air flowing through the duct, there is a constant need to provide an inexpensive method of making an electric heater having multiple stages of heat such that each stage provides some heat to each side of a support plate. In the prior art of open coil heaters having heater coils supported by ceramic insulators held in metal plates, one method of providing two stages of heat is to have one heater coil completely assembled on one side of the plate and the second coil on the opposite side, see U.S. Pat. No. 7,154,072. Upon energizing the first stage of heat, only the air on one side of the plate is heated making for a less than desirable heat distribution for the first heating stage.
Another method to improve heat distribution is to route the first stage coil so a portion of the heater coil is on one side of the support plate with the remainder of the coil routed on the opposite side, see U.S. Pat. No. 7,075,043 as one example. When these types of heaters are energized, heat is supplied to both sides of the duct during first stage heating. The second heat stage coils are similarly assembled to complement the first stage. This is an expensive design, as the ends of the heating element wire must be covered with special designed ceramic tubes or ceramic beads for electrical isolation to prevent grounding or reduction of electrical clearance, see the insulating members 36 in FIG. 1 as an example. Some designs use special designed ceramics to secure the heating element wire ends to prevent shorting, grounding or the reduction in electrical clearance as the wires are routed to terminals. A well accepted method long used is to provide individual termination points located immediately adjacent to the element coil ends. This is an expensive alternative, as power connections must be routed to multiple locations. Also, it is often impractical as some terminal locations may require power connections be made in excessively hot areas resulting in rapid deterioration under heat. Therefore, there is a need in the industry for a two stage, open coil electric heater that is inexpensive and has an arrangement wherein the first stage of the heater heats both sides of the air duct with the second heating stage complementing the first.
In the prior art there are usually either threaded style bolts or studs or blade or quick connect termination for power connection. Crimp style terminals made of flat metal stock for blade or quick connect termination crimped around resistance ends is well known and is presently sold by the TYCO Corporation. In the prior art, it is a common practice when bolt and threaded stud terminal style terminals are required for power connection, that these terminals are attached to element wire ends by welding, crimping, or pressure connection.
Welding is usually done by first mechanically staking the element wire ends into a slot in the head of a terminal bolt and then welding the two together. Crimping heating element wire ends to threaded bolts is accomplished by creating a tube style opening in one end of threaded stud terminals, inserting the heating element wire ends into the tube openings, and then mechanically closing the tubes so as to create a crimp connection. The least desirable connections are pressure connections in which resistance wire coil ends are looped around terminal bolts or threaded studs, then “sandwiched” between a combination of washers and nuts, whereby subsequent tightening of the nuts create electrical connections.
In the prior art, heating elements made as above are routed and assembled into the intended positions with heavy termination bolts attached to the coil ends. When a common threaded terminal power connection is needed, as for two stage or other multiple stage heaters, common element wire ends share a common terminal bolt or stud. When this type of connection is needed, the various methods of connection described above are followed except two or more element wire ends are connected to the required common terminal. For the welded connection, two or more common element wire ends are placed in the terminal bolt slot, mechanically staked then welded as above. For the crimp method, two or more common element wire ends are placed into the tube opening and crimped as above. For the pressure connection method, two or more common element wire ends are looped together then “sandwiched” as above and the termination completed. Thus, for the three prior art termination methods above, at least one end each of heater wire elements of multiple stage heaters share at least one common terminal bolt.
A shortcoming with respect to the termination of heater coils is that when threaded stud or bolt style termination for heaters is needed, prior art methods require the heating element wire ends to be first secured to heavy and cumbersome terminal bolts; the coil and terminal bolt assembly routed and subsequently secured to the coil support insulators. If the pressure connection method is used so as to allow heating element coils to be first assembled into a heater and then to connect to terminal bolts or threaded studs, this process is cumbersome and labor intensive. Also pressure electrical connections depend too much on the manual skill and attention of the person performing the task unlike a mechanical connection and thus generally are avoided if possible.
When threaded style terminations are required in the industry, there is needed a means to first make secure electrical connections between resistance wire coil ends and lightweight, easy to handle connectors that can later be attached to the terminal bolts or threaded studs whichever is used.
The present invention responds to the needs identified above by providing an improved open coil electrical resistance heater. The improved heater configuration that overcomes the problems noted above, by especially providing equal proportioning of the heater stages on either side of the support plate dividing the heater while at the same time arranging the terminal portions of the heater coils in the same location of the heater to reduce the need for extra supports and/or insulation and improving the manner of termination involving these types of heaters.